The birth of Monolithic Microwave Integrated Circuit (i.e. MMIC) technology constituted a great advance in microwave hardware and the communication tasks performed by such hardware. MMIC devices are analog electronic circuits formed (typically) in gallium arsenide monolithic chips, and capable of operating at conventional microwave frequencies and above. With this technology, one could implement microwave circuits ranging in size from a table top box all the way down to the size of a pack of playing cards, which had before been the size of multiple freestanding equipment racks. Not only did this save size, but it also saved weight. This is a matter of considerable importance to aircraft which use microwave equipment because even more than size, the weight which the aircraft must carry is the most important factor limiting its performance. So well received has MMIC technology been that industry has developed extensive suites of standard chips ranging in complexity to simple circuit elements all the way to sophisticated programmable microprocessors.
One of the most basic applications of microwave circuitry is the radio receiver, which not only permits communication with an aircraft's pilot, but also can perform a myriad of electronic warfare functions. For this, the conventional heterodyne receiver is well-suited, and MMIC chips are well-suited to the fabrication of such receivers, with one drawback. Although MMIC chips can readily implement high or low pass filters, and by combining the two can readily implement narrowband filters, it cannot implement narrow passband filters. A heterodyne radio typically receives a desired signal via a narrow band (i.e. tuned) filter, mixes the signal with a local oscillator, and then passes the mixed signal through a filter with a sharp and narrow passband in the vicinity of the local oscillator's frequency. This filter helps remove unwanted noise which may have been near enough in frequency to the desired signal to have passed through the narrow band input filter, but, as importantly, also filters undesired spurs and images of the desired signal which were created by nonlinearities in the mixing process, and by other electronic devices in the receiver. Current MMIC technology cannot implement such a sharp, narrow, passband filter. Without such filtering, one will at best be left with a noisy signal, and at worst a signal buried in noise. Worse still, if one has an application requiring several mixers, the noise introduced at each stage is cumulative.